Shijie Xie

A possible anthracene-based optical molecular switch driven by a reversible photodimerization reaction

By applying nonequilibrium Green’s function formalism combined with first-principles density functional theory, we investigate the electronic transport properties of an anthracene-based optical molecular switch. The molecules that comprise the switch can convert between the monomer and dimer forms upon photoexcitation, and two forms can keep stable over a wider temperature range. The transmission spectra of two forms are remarkably distinctive. Theoretical results show that the current through the monomer form is significantly larger than that through the dimer form, which suggests that this system has attractive potential application in future molecular switch technology.

Spin-current rectification in an organic magnetic/nonmagnetic device.

We propose a spin diode based on an organic magnetic co-oligomer or a magnetic/nonmagnetic heterojunction structure. The current and its spin polarization in the device are calculated with the spin-dependent Landauer-Büttiker formula. It is found that, by reversing the applied bias, the charge current and the spin current (SC) may be rectified at the same time or separately. A normal charge-current rectification usually takes place if the spatial electric structure is asymmetric. While a spin-current rectification may appear in two forms or their combination: one is that the spin-polarized orientation keeps unchanged but the magnitude of the SC is asymmetric with the bias; another is that only the spin orientation of the SC flips when the bias is reversed. By designing a suitable organic spin device, either of the two kinds of spin-current rectifications is obtained in our calculations. Finally, the effects of the properties of the organic interlayer and the structural asymmetry on the rectification are discussed.

Bias-induced orbital hybridization in diblock co-oligomer diodes

We investigate current rectification in diblock co-oligomer diode molecules on the basis of the Su-Schrieffer-Heeger model [Phys. Rev. B 22, 2099 (1980)] combined with the nonequilibrium Green’s function formalism. The current rectification observed in experiment [M. K. Ng et al., J. Am. Chem. Soc 124, 11862 (2002)] is well explained by the mechanism of bias-induced asymmetric hybridization of molecular orbitals. The positive bias tends to delocalize molecular orbitals through the hybridization, which produces a sharp increase in current at the threshold voltage; while the negative bias enhances the mismatch of energy levels and has no effect on the hybridization and the current.

Charge-transfer polaron induced negative differential resistance and giant magnetoresistance in organic spin-valve systems

Based on the static polaron Su–Schrieffer–Heeger model and the nonequilibrium Greens function formalism, we investigate the negative differential resistance (NDR) effect in organic spin-valve systems at low temperature and interpret it with a self-doping picture. A giant negative magnetoresistance exceeding 300% is theoretically predicted as the results of the NDR effects.

Length-dependent inversion of rectification in diblock co-oligomer diodes

The rectifying direction of diblock co-oligomer molecular diodes is investigated theoretically by analyzing the asymmetric bias effects on the molecular orbitals. The results reveal two competitive mechanisms in determining the rectifying direction, asymmetric energy shift of eigenstates and asymmetric spatial localization of wave functions upon the reversal of bias voltage. It is demonstrated that the dominated mechanism may be converted between the two mechanisms by changing the molecular length, which induces an inversion of the rectification. This work indicates the relative orientation of the two moieties is not sufficient to decide the rectifying direction of co-oligomer diodes.

Investigation on organic magnetoconductance based on polaron-bipolaron transition

We explore the magnetoresistance (MC) effect in an organic semiconductor device based on the magnetic field related bipolaron formation. By establishing a group of dynamic equations, we present the transition among spin-parallel, spin-antiparallel polaron pairs and bipolarons. The transition rates are adjusted by the external magnetic field as well as the hyperfine interaction of the hydrogen nuclei. The hyperfine interaction is addressed and treated in the frame work of quantum mechanics. By supposing the different mobility of polarons from that of bipolarons, we obtain the MC in an organic semiconductor device. The theoretical calculation is well consistent to the experimental data. It is predicated that a maximum MC appears at a suitable branching ratio of bipolarons. Our investigation reveals the important role of hyperfine interaction in organic magnetic effect.

Stability of hydrogenated graphene: a first-principles study

In order to explain the disagreement between present theoretical and experimental investigations on the stability of hydrogenated graphene, we have systematically studied hydrogenated graphene with different configurations from the consideration of single-side and double-side adsorption using first-principles calculations. Both binding energy and formation energy are calculated to characterize the stability of the system. It is found that single-side hydrogenated graphene is always unstable. However, for double-side hydrogenation, some configurations are stable due to the increased carbon–carbon sp3 hybridization compared to single-side hydrogenation. Furthermore, it is found that the system is energetically favorable when an equal number of hydrogen atoms are adsorbed on each side of the graphene.

Spin-dependent current modulation in organic spintronics

We investigate the spin-dependent current modulation in a model organic semiconductor sandwiched by two ferromagnetic electrodes. When the conductance band of the system is activated by an applied bias voltage, the majority-spin electrons are successively blocked within the organic semiconductor and form nonequilibrium polarons. This majority-spin blockage will modulate the minority-spin current due to the effective spin-spin coupling mediated by the electron-phonon interaction. This study suggests that the spin-blockage induced current modulation is a rather robust phenomenon in organic spintronics.

Single-molecule photon emission statistics for systems with explicit time dependence: Generating function approach

Generating function calculations are extended to allow for laser pulse envelopes of arbitrary shape in numerical applications. We investigate photon emission statistics for two-level and V- and Lambda-type three-level systems under time-dependent excitation. Applications relevant to electromagnetically induced transparency and photon emission from single quantum dots are presented.

Charge-induced spin polarization in thiophene oligomers

Charge-induced spin polarization in small organic molecules is a key factor for spin transport and magnetic effects in related organic devices. In this work, we study spin polarization in charged thiophene oligomer molecules by calculating the magnetic moment with density functional theory. It is found that the emergence and variation of the net magnetic moment is related to both the amount of charge injected and the polymerization of the oligomer. In combination with model analysis, we conclude that the strong electron–electron interaction and electron–lattice interaction in organic materials are responsible for charge-induced spin polarization.